The present invention relates to an electronic circuit for communicating data onto a bus. More particularly, the invention involves an electronic transmitter circuit which utilizes matched current sources to synchronously drive a pair of serial bus lines.
The circuit of the present invention is particularly suited to the needs of a multiplexed bus operating in an automotive vehicle environment. In such environment, the circuit generates digital signals which are driven onto a complementary pair of bus lines. In keeping with system design constraints, the circuit output is functionally equivalent to a matched pair of synchronized current sources respectively pushing and pulling the pair of bus lines. In further keeping with the mandates of the application, the circuit is configured with current drivers which minimize bus radio frequency (RF) radiation and includes features suitable to detect and appropriately respond in a ground loss operating environment.
The basic requirements of the composite system to which the present circuit relates have been the subject of numerous publications. For example, some requirements are set forth in Engineering Standard No. PF-8219, issued by Chrysler Corporation in approximately June, 1986 under the title "Integrated Circuit--Universal C.sup.2 D Differential Serial Bus Interface". Further early development is set forth in the article by Miesterfeld, entitled "Chrysler Collision Detection (C.sup.2 D)--A Revolutionary Vehicle Network", which appeared as SAE Technical Paper Series, 860389, dated Feb. 24-28, 1986. A recent discussion of the operational system requirements appeared in the article by Fassnacht et al. entitled "Chrysler Collision Detection (C.sup.2 D.TM.) Bus Interface, Integrated Circuit User Manual" which was published as SAE Technical Paper Series, 880586, in approximately Feb. 29-Mar. 4, 1988. The requirements of the operating environment are also extensively documented in recently issued U.S. Pat. No. 4,739,323, the subject matter of which is now incorporated by reference.
What remains as undefined within the various references is the implementation of a transmitter capable of generating the synchronous and matched current sources with suppressed RF generation and ground loss operation capability. Moreover, it is particularly important that the transmitter circuit be suitable for fabrication as a CMOS type integrated circuit, in that the various transceiver (transmitter/receiver) functions will eventually be merged onto a common semiconductor chip. Other aspects of the serial data bus operating environment to which this present transmitter is peculiarly suited are detailed in the various references identified hereinbefore.
The application of the current mirrors to field effect transistor integrated circuits per se is relatively well-known. For instance, a sensing circuit with multiple current mirror connected field effect transistors is described in U.S. Pat. No. 4,488,065.